kstd: implement fixed-size ring_buffer

This patchset introduces a (non-standard) container to the kernel standard
library. The new container, `kstd::ring_buffer<T, N>`, is a fixed-size ring
buffer, with an LRU overwrite policy. This means, that the size of the buffer
is defined at compile-time. When a buffer is full, pushing further elements
into it will replace existing ones in a Least Recently Used manner.

The first target application of this new container is an extension to the
existing logging system, providing a kernel log buffer.

See merge request teachos/kernel!62